1. Field of the Invention
The present invention relates to a switching power source control circuit for controlling the output value of a switching power source to be a target value by thinning-out switching pulses.
2. Description of the Prior Art
As a conventional example of thin-out control, a so-called on-off control method for controlling AC power of a commercial power source frequency is known. As AC power control in a commercial frequency, phase control by means of a discharge tube, an SCR (silicon controlled rectifier or thyristor), a TRIAC (triode AC switch), and so on is mainly used.
Phase control has a sufficient control capability but produces remarkably large noise because a switching element is turned on in a phase other than a phase where a current (or voltage) is zero. In order to reduce such a disadvantage, therefore, in thin-out control, a switching element is turned on (or turned off) at a point of time of zero-crossing of the current of the switching element to thereby properly adjust the ratio of the number of turn-on cycles to the number of turn-off cycles to control electric power to be supplied to a load. As a load, in most cases, an electric room-heating device, a cooking device, an electric furnace or the like is used, and as the switching element, an SCR, a TRIAC or the like is used.
The diagrams (a), (b) and (c) of FIG. 15 show typical examples of waveforms in the case of phase control in AC power control, in the case of ideal thin-out control and in the case of conventional thin-out control by on-off control, respectively. Each of the examples shows the case where electric power is reduced to a half of its maximum. In the phase control shown in FIG. 15(a), turn-on is made in the vicinity of the maximum current value, and there arises a problem that intensive noise is produced to disturb other devices.
FIG. 15(b) shows an ideal waveform in the case where electric power is reduced to a half by repeating turning-on and turning-off on alternate cycles. Because it is generally difficult to realize such thin-out control, the method shown in FIG. 15(c) is used in most cases in the present state. Although FIG. 15(c) shows the case of three continuous on-cycles and three continuous off-cycles for simplification of the drawing, there may be a case where the number of continuous on- and off-cycles reaches the order of tens of cycles in practical use in accordance with the time constant of an electric furnace or a temperature measuring sensor.
In the method shown in FIG. 15(c), however, if the number of continuous on- and off-cycles increases, the response in power control becomes slow as a result to thereby lower the control accuracy compared with the ideal method shown in FIG. 15(b) even if the thin-out rate is fixed between the both methods. In order to improve the control accuracy, therefore, it is necessary to conduct not only proportional (P) control but also complex control such as proportional-plus-derivative (P,D) control in which derivative control is combined with proportional control or integral-plus-derivative (I,D) control in which integral control is combined with derivative control. In some case, it is required to conduct further complex control such as proportional-plus-integral-plus-derivative (P,I,D) control.
For example, in the case of control on an electric furnace, the increase of the number of continuous on- and off-cycles in a commercial frequency is caused by the size of the heat capacity of the electric furnace or the time lag of detection of temperature. Accordingly, the time lag in an error amplifier or the like may be neglected sufficiently as long as the error amplifier is carried out with the speed of an ordinary differential amplifier (operational amplifier).
In the case where conventional on-off control is applied to switching power source control, however, continuation of turn-on or turn-off of a switching element is caused by time lag of a smoothening circuit in an output stage and by time lag of an error amplifier. As a result, there arises a problem that output variation longer in period compared with a switching frequency becomes large or the control becomes unstable. In addition, though switching frequencies used in the recent switching power source are generally in a range of from 100 to 200 KHz, switching frequencies in a range of from 500 KHz to the order of MHz has been reported with the advance of use of higher frequency recently. In such a case, the aforementioned problem becomes serious.